1. Field of the Invention
This invention pertains generally to optical focusing reflectors and lenses, and more particularly to high contrast gratings configured as planar focusing lenses and reflectors.
2. Description of Related Art
Numerous optical devices require the inclusion of one or more focusing reflectors and/or lenses for proper functioning. Difficulties arise with integrating these optical lenses, and reflectors, requiring precisely curved surfaces and optical properties into devices, such as semiconductor devices, that may include vertical cavity surface emitting lasers (VCSEL) and the like. Various techniques have been proposed for simplifying these fabrication processes, such as fabricating standing structures around emitting areas of multiple VCSELs and using these structures for retaining ball lenses or other optical elements. It is also necessary to provide alignment of these optical elements with the remainder of the device structure. Additionally, numerous problems arise with the thickness of conventional lenses and reflectors being integrated within optical devices.
Focusing reflectors and lenses are perhaps the most fundamental components in applications involving manipulation of light, including imaging, communications, display, sensing, solar cells and measurements. Systems and devices in all these areas benefit from monolithic integration and the corresponding decreases in size, weight, and costs. Integration of focusing elements requires a design which is compatible with standard microfabrication processes while offering comparable or better performance relative to bulk optics.
Of all the parameters of a reflector or lens, numerical aperture (NA) is perhaps most critical because it indicates the focusing or resolving power of a reflector or lens. The focusing/defocusing capability of conventional simple lenses arises from the shape of the lens and the index contrast between the lens material and air. Because the lens must be transparent, the choice of material is limited and the highest refractive index is approximately two, which limits the NA of conventional lenses. Conventional focusing reflectors are typically curved mirrors made of glass coated with metal. The focusing capability of a reflector arises from its aspherical shape, enabling high NA. However, both the lens and reflector require an aspheric shape and bulky thickness, presenting difficulties for standard microfabrication techniques.
Zone plates and Fresnel lenses are attractive alternatives to simple lenses because they are planar and compact. A zone plate consists of a set of radially symmetric rings which alternate between opaque and transparent, harnessing diffraction to create a lensing effect. The major drawback of zone plates is that they absorb a significant part of the input power, making them undesirable for any optical application where low loss is required.
Fresnel lenses are a planar alternative which consists of many concentric segments having continuous height variations. The quality of a Fresnel lens depends on the number of segments and the accuracy of the height variation. However, at the microscales necessary for device fabrication, the creation of Fresnel lenses becomes difficult. So although high NA is achievable with a Fresnel lens, incorporating Fresnel lenses within integrated optics applications is problematic.
Accordingly, there is a need for small, light, and inexpensive focusing structures (lenses and reflectors) which can be readily integrated within semiconductor circuit devices. The present invention fulfills that need and others, while extending the range of applications into which the lenses and reflectors can be cost-effectively integrated.